1. Field of the Invention
The present invention relates to a clutch control device for a vehicle with an automatic transmission, and more particularly to a technique for relieving shock when return from fuel cut is attained regardless of a transmission schedule while improving the effect of engine brake due to fuel cut during coast-down or braking under the state with a high transmission ratio (transmission ratio.gtoreq.1), and preventing engine stall.
2. Description of the Prior Art
A CVT (continuously variable transmission) mounted on a vehicle has been proposed in Unexamined Japanese Patent Publication 1-119433 (1989). The vehicle equipped with such a CVT compares a vehicle speed Vs with a target vehicle speed Vso during coast down or braking, and when Vs.ltoreq.Vso, changes the clutch from the direct coupling state into the incomplete clutch or semi-coupling state. In this case, the target vehicle speed Vso is set at a predetermined value (e.g. 25 km/h).
The means for automatically performing such a clutch function includes a lock-up clutch controlled by an automatic control device, a hydraulic single-plate clutch, or the like.
In the above clutch control device for a vehicle equipped with an automatic transmission, a change from the direct coupling state into the semi-coupling clutch state is decided by a predetermined value in terms of the vehicle speed of Vs. For this reason, depending on some transmission schedules, when return from fuel cut occurs in the direct coupling state of a clutch during coast down or braking, engine brake force is reduced. A change in the driving torque when the return from fuel cut occurs is directly passed to the vehicle, thus producing shock disadvantageously. Particularly, when the transmission ratio is as high as 1 or more because of a certain transmission schedule, great shock due to return from fuel cut occurs.
In abrupt braking (e.g. braking on a low .mu. passageway), the shift to the semi-coupling state is delayed, thereby producing engine stall disadvantageously.